[Docs] Document periodic tasks

docs/admin/02_install.rst

@@ -125,6 +125,8 @@ from `yarnpkg`, collect static files, and migrate the database to the final sche
    aleksis-admin migrate
    aleksis-admin createinitialrevisions
 
+.. _core-configure-uwsgi:
+
 Configure uWSGI
 ~~~~~~~~~~~~~~~
 

docs/admin/07_tasks.rst

+Background tasks
+================
+
+Operations that are expected to take a long time are run as background tasks.
+For this, at least one `Celery`_ worker has to be running, e.g. by coupling it
+with uWSGI as laid out in :ref:`core-configure-uwsgi`.
+
+If a task is triggered from the AlekSIS frontend, for example by starting an import
+job, a progress page is displayed, and the result of the job is waited for.
+
+.. _core-periodic-tasks:
+
+Periodic tasks
+~~~~~~~~~~~~~~
+
+Some tasks are also run on a schedule. For example, the backup job is run on
+a regular basis.
+
+All tasks in AlekSIS that are expected to run have a default schedule, which
+is registered when migrating the database. Changing this default schedule
+is currently only possible through the Django Admin backend, under
+*Admin → Backend Admin*.
+
+Under the *Periodic Tasks* app, you can define schedules and tasks. The names
+of tasks you can add manually are documented in the respective sections
+of the manual.